Reactions heterogeneous catalyzed hydrogenation

The commonly accepted mechanism of heterogeneously catalyzed hydrogenation involves activation of both the hydrogen and the C—C multiple bond adsorbed on the metal surface. First one hydrogen atom is transferred to the least hindered position of the multiple bond to give a half-hydrogenated adsorbed species. This reaction is fully reversible and ac-... 

Heterogeneously catalyzed hydrogenation of alkenes is generally considered to be a structure-insensitive reaction, as was deduced from numerous studies on more or less complex model catalyst systems [40-54]. However, the following sections will give examples of the opposite case. 

Heterogeneously catalyzed hydrogenation is a three-phase gas-liquid-solid reaction. Hydrogen from the gas phase dissolves in the liquid phase and reacts with the substrate on the external and internal surfaces of the solid catalyst Mass transfer can influence the observed reaction rate, depending on the rate of the surface reaction [15]. Three mass transfer resistances may be present in this system (Fig. 42.1) ... 

In recent years, attempts have been made to make use of the advantages of the supercritical carbon dioxide in chemical reactions. The first technical examples concerning the use of carbon dioxide in a pilot-plant scale chemical reaction were heterogeneous catalyzed hydrogenation and radical polymerization [38-42]. Meanwhile, hydrogenation reactions have been scaled up in a 1000 t/a commercial multipurpose plant. 

In a very recent example (Figure 1.53), INEPT DOSY has been applied to confirm the dinudear nature of the unstable Zirconium intermediate 141 in the reaction of CO with [ZrHCl(Cp)], 140 [344], a model reaction for the heterogeneously catalyzed hydrogenation of CO2 to methanol [345]. As was expected for a binuclear compound, the diffusion coefficient of intermediate 141 is smaller than for the mononuclear 142. 

Catalytic hydrogenation represents a set of reactions that will be extremely important in the production of value-added chemicals from biomass. Already now, they play a significant role in today s industry, and holds great promise for further developments. Here, selected examples of heterogeneously catalyzed hydrogenations of chemicals available from renewables resources are presented. 

Heterogeneously catalyzed hydrogenation reactions can be run in batch, semibatch, or continous reactors. Our catalytic studies, which were carried out in liquid, near-critical, or supercritical C02 and/or propane mixtures, were run continuously in oil-heated (200 °C, 20.0 MPa) or electrically heated flow reactors (400 °C, 40.0 MPa) using supported precious-metal fixed-bed catalysts. The laboratory-scale apparatus for catalytic reactions in supercritical fluids is shown in Figure 14.2. This laboratory-scale apparatus can perform in situ countercurrent extraction prior to the hydrogenation step in order to purify the raw materials employed in our experiments. Typically, the following reaction conditions were used in our supercritical fluid hydrogenation experiments catalyst volume, 2-30 mL total pressure, 2.5-20.0 MPa reactor temperature, 40-190 °C carbon dioxide flow, 50-200 L/h ... 

All one-step additions to C=C double bonds are mechanistically required to take place cis-selectively (ds > 99 1) (Sections 3.3.1-3.3.3). In addition, the heterogeneously catalyzed hydrogenation of alkenes also usually takes place with very high cw-selectivity, in spite of its being a multistep reaction (Section 3.3.4). 

The addition of hydrogen to olefins is made possible only with noble metal catalysts. They allow for a multistep, low-energy reaction pathway. The noble metal catalyst may be soluble in the reaction mixture in that case we have a homogeneously catalyzed hydrogenation (cf. Section 14.4.7). But the catalyst may also be insoluble then we deal with a heterogeneously catalyzed hydrogenation. 

Reactions on Metal Surfaces Heterogeneously Catalyzed Hydrogenation and Isomerization... 

Discussion Point DPI While there are many examples of industrial processes that depend on heterogeneously catalyzed hydrogenation reactions, there are very few (apart from the enantioselective processes) which are homogeneously catalyzed. Suggest some reasons for this. 

One synthesis approach towards y-hydroxylated ionic liquids is a Michael-type addition of a protonated ammonium salt to a a, /l-an sal uni led carbonyl compound such as methylvinyl ketone yielding an oxobutyl functionalized cation. Intrinsically unstable due to retro-Michael reaction, this OS could however be transformed by a heterogeneous catalyzed hydrogenation reaction yielding a hydroxyl functionalized TSIL [24] (Fig. 7). 

Step a involves a Friedel-Crafts acylation where HF serves as the catalyst and the solvent for the reaction. Although HF is highly toxic, it can be contained on an industrial scale and completely recycled. The acetic acid by-product can also be recycled. The second step (b) involves heterogeneously-catalyzed hydrogenation of a carbonyl group to give the corresponding alcohol 80 with an atom economy of 100%. Either palladium on charcoal or Raney-nickel can be used as a catalyst. Step c is an excellent example of an alcohol carbonylation that we saw earlier in Section 9-5 this process, like carbonylation of methanol, has an atom economy of 100%. The overall atom economy of the new process, which... 

The structure of substrates may affect reactivity in heterogeneously catalyzed hydrogenations by affecting the rate constant of the reaction, and... 

We shall discuss the process for the example of the hydrogenation of benzalde-hyde in various reactors [15]. The heterogeneously catalyzed hydrogenation of ben-zaldehyde is a model reaction for the hydrogenation of aromatic aldehydes. The main reactions are shown in Equation 13-19. 

ATR-FTIR/UV-vis/Raman spectroscopy has been applied for mechanistic studies of the heterogeneously catalyzed hydrogenation of imines and of formal [3+3] cyclocondensation reactions catalyzed by Lewis adds [11, 33]. In the latter reaction, different products were obtained dependent on the Lewis acid used. It could be elucidated by in situ spectroscopic investigations that the intermediate complex formed between the ketene acetal component and the Lewis acid (an acid-catalyzed rearrangement reaction occurs in the case of trimethylsilyltrifluoromethanesulfonate (TM SOTf), forming a bischelate complex with TiCl4) controls the subsequent attack of the second reactant in the form of a diene, and consequently, the mechanism of product formation [33]. 

Another report of hydrogenation under SCF conditions from the patent literature includes claims for a continuous heterogeneously catalyzed hydrogenation reaction process by Subramaniam and Said (182,183). The primary focus of these patents is the in situ mitigation of coke buildup in porous catalysts, but an SCF-mediated hydrogenation process is a cited application (and claim). 

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